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Need help with foul odor from bathroom sink

When water is turned on in sink the foul odor seems to come from the overflow. I have checked the p trap and it seems to be alright. What could be causing this problem?

Yes, the sink is vented and yes I have removed the ptrap. From the ptrap there is a 45 then a 90 going to the rooftop. The ptrap had water in it to prevent the smell so I can't understand how it's getting through. The sewer gas is definitely getting through some way.

Need help with foul odor from bathroom sink

Need help with foul odor from bathroom sink

Does the sink gurgle when the water flows down the drain?
If the vent is plugged then the water will gurgle.
You may have some other debris that is in the overflow. Put some baking soda in the overflow and then pour some vinegar down after. This will foam up and it may clean out the overflow. Follow this up with some hot water to flush out the overflow.

Need help with foul odor from bathroom sink

Tried the baking soda and vinegar trick already, didn't work. I filled the sink full of water and the flow from the overflow seems alright. Also, there is no gurgling when water is released. Ever once and again the drain becomes slow but after a couple plunges it's alright for a couple months. The drain that is, the odor never leaves. Any other thoughts?? I'm stumped....